A conventional ultrasonic probe comprises a transducer package which must be supported within a probe housing. As shown in FIG. 1, a conventional transducer package 2 comprises a linear array 4 of narrow transducer elements. Each transducer element is made of piezoelectric ceramic material. The piezoelectric material is typically lead zirconate titanate (PZT), polyvinylidene difluoride, or PZT ceramic/polymer composite.
Typically, each transducer element has signal electrodes and ground electrodes formed on opposing faces thereof. The signal electrodes can be connected to a signal source via conductive traces on a flexible printed circuit board (PCB) 6.
The transducer package 2 also comprises a mass of suitable acoustical damping material having high acoustic losses, e.g., silver epoxy, positioned at the back surface of the transducer element array 4. This backing layer 8 is coupled to the rear surface of the transducer elements to absorb ultrasonic waves that emerge from the back side of each element so that they will not be partially reflected and interfere with the ultrasonic waves propagating in the forward direction.
Before installation into the probe housing 2, the transducer element array 4, flexible PCB 6 and backing layer 8 are bonded together in a stack-up arrangement which is secured inside a four-sided array case 10, i.e., a "box" having four side walls but no top or bottom walls. The array case protects the fragile transducer elements during probe assembly.
Typically, a first acoustic impedance matching layer 12 is bonded to the bottom the transducer stack-up, as shown in FIG. 1. A second acoustic impedance matching layer 14 is later bonded to the first acoustic impedance matching layer 12. The acoustic impedance of the second matching layer must be less than the acoustic impedance of the first matching layer and greater than the acoustic impedance of the human body or water. For example, the second matching layer 14 may consist of a plastic material, such as polysulfone, which has excellent acoustic transmission properties.
The front face of the second or outermost acoustic impedance matching layer is conventionally bonded to the planar rear face of a convex cylindrical lens 16 using an acoustically transparent thin layer of silicone adhesive. Lens 16 serves three purposes: (1) acoustic focusing (due to its lens-shaped cross section and its low acoustic velocity material properties); (2) providing a chemical barrier to protect the transducer elements from attack by gels, body fluids, cleaning agents, etc.; and (3) providing an electrical barrier to protect the patient from the electrically active transducer elements. The lens is conventionally made of silicone rubber.
Typically the silicone rubber lens 16 is bonded to the plastic matching layer 14 by a layer 18 of silicone adhesive, as depicted in FIG. 2. However, it is difficult to achieve a high-quality adhesive bond between the silicone rubber lens and the plastic matching layer because these materials have very different adherability characteristics. This presents a problem because an intimate bond between the silicone rubber lens and the plastic matching layer is required in order to produce the desired acoustic transmission characteristics and mechanical stability.